[SDTEST-997] parallel tests gem support

[anmarchenko]

What does this PR do?
Adds proper support for the parallel_tests gem (when used with RSpec): now when running tests with parallel_rspec command datadog-ci will trace and report a single test session. Also, the worker processes will now share the remote configuration and other backend data between them, in order to save a lot of time and fire fewer requests to the Datadog's backend.

Motivation
When parallel_tests gem is used, we want to report a single test session to correctly calculate time and cost savings from Datadog Test Optimization product

Additional Notes
The most important changes are:

• new parallel_tests subfolder in contrib: it instruments ParallelTests::CLI class to start test session and test module when CLI is invoked
• new internal configuration test_visibility_drb_server_uri that is used to propagate DRb (distributed ruby) socket URI between worker processes via ENV variable
• rspec instrumentation no longer stops test session and test module if it is running under parallel_tests
• datadog-ci components (Remote, TestVisibility, TestManagement, TestOptimisation) now dump their global state to a file if started under parallel_tests. Worker processes then read this state from a file to avoid going to Datadog backend unnecessary. See utils/file_storage.rb file for the implementation of this storage interface. See utils/stateful.rb for a concern that adds ability to store internal state to any component.
• introduced a new test suite tracing mode for test visibility: local_test_suites - when enabled, the test suites are traced in the local process (worker). This mode is enabled by default. Minitest instrumentation, on the other hand, always disables this mode because minitest split tests by individual test, not by test suite (test file). This is done for performance reasons because IPC increases the overhead of tracing.
• test session context and test module context are now copied from remote process to each of the worker process. It includes span ID, name, test service and some tags that are shared between all span levels.

How to test the change?
Manual testing using anmarchenko/rubocop project, results:

Added unit tests to make sure that ParallelTests::CLI opens test_session and test_module